Forensic medical examination has shown that the 58 year old General-major Igor Sergun died of acute heart failure.

The
doctors who conducted the autopsy on the body of General Sergun, who
died on the 3rd of January in the Moscow region, made preliminary
conclusions about the causes of his death. According to sources familiar
with the situation, the doctors concluded that the body of 58-year-old
man was overworked.

"One
of the reasons for his death was fatigue: over-processing, not getting
enough sleep were all accompanying signs and "symptoms," commented a
source in medical circles.

However, as it became known to LifeNews, the official cause of death was called acute heart failure.

The
head of the Main intelligence Directorate of the General staff of the
Armed forces of the Russian Federation, Igor Dmitriyevich Sergun died on
the 3rd of January in his house in the Podolsk district of the Moscow
region.

Not very informative, hence our headline.

Stratfor has a look from a different angle:

A Mysterious Death Raises Questions in Russia

Intrigues within the Kremlin reignited Monday after the chief of
Russia's military intelligence service, Igor Sergun, died unexpectedly.
Sergun was a relatively unknown figure who kept a very low profile over
his 30-year career, despite the fact that his position at the head of
the Main Intelligence Directorate (GRU) of the General Staff of the
Armed Forces made him one of the most powerful figures in Russian
security.

Sergun came to power in 2011, at a time when the GRU's position was
under attack by the Federal Security Service (FSB) and the Foreign
Intelligence Service — both of which were attempting to gain control of,
or at least influence in, intelligence operations in Russia's
borderlands. Sergun was able to consolidate the GRU, fortifying the
military intelligence service's position among the security groups.

Russia's various security services have long vied with one another
for power. Even though Russian President Vladimir Putin served in the
FSB (known at the time as the KGB), he has not always let the agency
have its way. Putin has tried to keep a balance among
the various services — a difficult feat in a world of intrigue and
espionage. That balance has been off for the past two years, mainly
because of events in Ukraine. Moscow's failure to anticipate the
overthrow of Ukrainian President Viktor Yanukovich and the installation
of a pro-West government in Kiev in early 2014 largely fell on the FSB.
The service reportedly was restructured by
mid-2014, and the GRU gained more responsibility for intelligence
inside Ukraine — a humiliation for the FSB. The GRU and FSB wrestled
with each other during the remainder of 2014 and all of 2015 over
control of ground intelligence in Ukraine.

Evidence of the
behind-the-scenes struggles could be seen in the turnover of pro-Russian
separatist leaders in eastern Ukraine and in sporadic reports from
Ukrainian intelligence.

Recent developments provide few details about the current state of
the power struggle. Four months ago, rumors circulated in Russian media
that one of the GRU's biggest backers, Vladislav Surkov, lost the
Ukraine portfolio once again. Surkov, alongside Sergun, had been
instrumental in implementing Russia's so-called hybrid warfare strategy
in eastern Ukraine and in coordinating the separatist leaders throughout
the year. Over the weekend, a Ukrainian intelligence source claimed
that the FSB was back in eastern Ukraine working with the separatist
leaders. This would indicate a strengthening of the FSB's position....MORE